Answer:
Technically, the US Constitution was the true founding point of the US. It was the final establishing moment that began with the tax protests and revolution. The former government of the union was based on the Articles of Confederation, which afforded too weak of a centralized government. It not only helped the US centralize power, law, and identity, but really defined it as a maturing nation and culture. No other nation had done that before. That is, defeat the British Empire and define an indepedent government and culture. Up until that time, democracy was an ancient ideal of the Greeks that had long been lost. It was the crown achievement of a long trail from the Magna Carta to the Renaissance to the Enlightenment. It helped the US create its own identity and completely separate from European Imperialist hegemony. In so doing, they fully realized the European potential. In a sense, it was like the barbarian conquerors of Rome realizing what Rome was about and applying it in their own way. It's quite strange. The US Constitution is a Greco-Roman-Judeo-Christian document.
It's like a child from a disfunctional family running away and becoming world famous and renouned for their amazing achievements. The Constitution is the internal dialogue that allowed that runaway to accomplish that.
Makes me wish I could move to Mars and help write that Constitution... It's all we got.